Lithium ion secondary batteries have been used for small information devices such as cellular phones, book-sized personal computers, and the like because they have a high energy density and can be operated at high potential. Lithium ion secondary batteries are each comprised mainly of a positive electrode, a negative electrode, and an electrolyte. Lithium complex oxide-based materials are used for the positive electrode, while lithium complex oxide-based materials, carbon materials, metal lithium, lithium alloys and the like are used for the negative electrode. For the electrolyte, for example, a non-aqueous organic electrolyte in liquid form (electrolyte solution) has been used widely. There is a demand for the development of lithium ion secondary battery materials having a higher capacity in anticipation of an increase in demand for large-sized, high-output, and long-life secondary batteries in future as power supplies for automobiles, capacity-enlarged stationary power supplies and the like.
Since large-sized batteries have become popular, research and development of sodium ion secondary batteries making use of sodium have been carried out instead of that of lithium ion secondary batteries making use of lithium which is expensive due to poor resources. The materials of positive and negative electrodes of sodium ion secondary batteries are required to be able to insertion/extraction of sodium ions, be highly reversible, and moreover, have a large sodium insertion/extraction amount. As the materials of a positive electrode of sodium ion secondary batteries, various oxide materials having a tunnel type structure or layered rock-salt type structure have already been reported from such a standpoint.
Examples of reports on negative electrode materials of sodium ion secondary batteries are not so many as those of reports on positive electrode materials and a test on a negative electrode material using metal sodium, sodium-tin alloy, soft carbon, or the like is performed at present (Patent Document 1). Industrial use of a sodium metal, which is more active than a lithium metal, as a negative electrode material cannot be recommended from the standpoint of safety and development of an oxide-based negative electrode material is important. The present inventors therefore payed attention to the crystal structure of Na0.44MnO2 used as a positive electrode material and having a one-dimensional tunnel type structure.
The tunnel space of Na0.44 MnO2 is presumed to have a shape facilitating insertion or extraction of sodium ions. From the standpoint of a potential difference, NaxTi4O9 (2≤x≤3) which is a sodium titanium oxide having, similar to Na0.44MnO2, a large one-dimensional tunnel type structure became a candidate for a negative electrode material. It however had the drawback that metal sodium was conventionally used for the synthesis of NaxTi4O9 (2≤x≤3). In addition, synthesis of a NaxTi4O9 (2≤x≤3) polycrystal has not yet been investigated.